A thin film transistor (hereinafter sometimes referred to as TFT), which can be formed on a substrate of an insulating material such as glass, is an important device in electronics technology.
As materials for channel layers in TFTs, amorphous silicon and polycrystalline silicon are now most widely used. In recent years, metal oxide semiconductors have been attracting attention as TFT channel layer materials which substitute for these silicon materials and research and development on such semiconductors are being carried out actively. Metal oxide semiconductors not only provide excellent features as channel layer materials but also are characterized in that they can be formed around room temperature. For this reason, they are considered as major candidates for the channel layer material for formation of a TFT on a flexible substrate like a plastic film.